1. Field of the Invention
This invention relates to a monocrystal rod which is utilized for producing semiconductor device, solar cell or the like.
2. Prior Art
Examples of methods extensively used for preparing a monocrystal rod such as a silicon semiconductor or compounds semiconductor include a Czochralski method, which comprises a step of pulling a seed crystal upwards from a melt in a crucible while rotating the seed crystal and the crucible in the same or opposite direction.
FIG. 1 shows an example of a monocrystal rod which is prepared by the method above. Numeral 1 denotes a seed crystal, and numeral 2 denotes a monocrystal rod which has a circular cross section. The monocrystal rod 2 includes a neck section 3 which is smaller in diameter than the seed crystal 1, a main rod section 4 which is cylindrical and whose diameter is the largest in the rod 2 and is equal to the diameter of the required rod, and a shoulder section 5 which is tapered for linking continuously the neck section 3 to the main rod section 4. Numeral 6 denotes a bottom section of the main rod section 4 which is tapered. Only the main rod section 4 in the rod 2 can be used for producing the semiconductor chips or the like, and its diameter is determined depending on the diameter of the required rod. Also, the diameter of the main rod section 4 generally varies slightly depending on the exactness or precision of techniques of controlling the size thereof. Therefore, it is preferably similar to the diameter of the required rod, and the variation in the diameter should be as small as possible in order to efficiently prepare the main rod section 4.
The neck section 3 is provided so as to reduce the dislocation density of the seed crystal 1 prior to pulling up the seed crystal 1 with a predetermined diameter and thus increase the completeness thereof. For example, when the rod 2 is made of silicon semiconductor, the neck section 3 is about 3 to 4 millimeters in diameter and about several ten millimeters in length. In this case, the tensile strength of the neck section 3 is at most about 100 kgw. However, even if the weight of the rod 2 is less than 100 kg, the neck section 3 will often be damaged, with the result that ultimately the rod 2 will fall down when the rod 2 is twisted, or when force is exerted on the rod 2 in its transverse direction. In case the rod 2 falls as the result of rupture of the neck section 3, the apparatus for pulling the rod 2 is damaged fatally, and moreover a security problem comes about because the rod 2 fallen is very heavy and hot. Therefore, the step of pulling the rod 2 needs to be concluded before the weight of the rod 2 grows too heavy for the neck section 3 to endure the breakage or before the tensile strength caused by the weight of the rod 2 becomes close to or above to such an extent that its maximal tensile strength of the neck section 3.
According to a conventional method, the rod 2 is pulled with care so that the weight of the rod 2 will remain at a value considerably below the weight at which the neck section 3 has the maximal tensile strength. When the weight of the rod 2 has reached a predetermined value the pulling operation is stopped and the thus-obtained rod 2 is cut out from the seed crystal 1, and then another rod 2 is pulled upward again with the seed crystal 1. Alternatively, according to another conventional method, the rod 2 is pulled upward while feeding a limited amount of starting silicon material into a melt in a crucible from the beginning with view to avoiding the above-described difficulty.
However, the above-mentioned conventional methods have a defect that the productivity is very low because the length of the rod 2 which can be pulled in a single operational step is limited. Moreover, the availability of the apparatus is very low.
Also, even if the rod 2 is pulled upwards with a sufficient reserve so as to make the weight of the rod 2 below a predetermined value which is determined depending on the mechanical strength of the neck section 3, a possibility cannot be denied that the neck section 3 is damaged and the rod 2 falls down as a result of unexpected causes such as earthquakes, mechanical shocks and the like. On the occasion whose the rod 2 falls, as aforementioned, the lower portion of the expensive apparatus which includes a crucible for containing a melt, a susceptor for surrounding the crucible, heating means for heating the crucible through the susceptor, and a cylinder for keeping warmth of the crucible and the susceptor together will be destroyed.